A fuel cell is an electrochemical cell which generates electricity by combination of hydrogen and oxygen. Unlike a general chemical cell such as a dry cell or storage cell, the fuel cell can generate electricity continuously as long as the hydrogen and oxygen are supplied. In addition, little heat loss occurs in the fuel cell so that efficiency of the fuel cell is twice as high as efficiency of internal combustion engine. Furthermore, since the fuel cell directly converts chemical energy generated by the combination of hydrogen and oxygen into electric energy, the fuel cell is eco-friendly and capable of mitigating the concerns about the exhaustion of fossil fuel.
Depending on the type of electrolyte, the fuel cell may be classified into a polymer electrolyte fuel cell, a phosphoric acid fuel cell, a molten carbonate fuel cell, a solid oxide fuel cell, and a alkaline fuel cell.
One of the most important factors to improve the performance of the polymer electrolyte fuel cell is to maintain the amount of the humidity contained in the polymer electrolyte membrane of a membrane-electrode assembly by continuously supplying predetermined amount of moisture to the membrane. This is because the generating efficiency of the fuel cell is rapidly deteriorated as the polymer electrolyte membrane is dried.
Among the various methods for humidifying the polymer electrolyte membrane is a method using a polymer separation membrane for supplying the moisture to the dry reaction gas.
The membrane humidifying method uses a membrane which allows only the vapor contained in the exhaust gas to pass therethrough, to thereby supply the vapor to the polymer electrolyte membrane. This method is advantageous in that a humidifier of a small size and light weight can be manufactured.
As to the membrane for the membrane humidifying method, it is preferable to use a hollow fiber membrane(s) which can provide a large permeation area per unit volume when applied in a module. That is, when a humidifier is fabricated with the hollow fiber membranes, the hollow fiber membranes having a large contact surface area can be highly integrated so that the fuel cell is sufficiently humidified even with small volume. The hollow fiber membranes can be formed of a low-priced material. Also, moisture and heat contained in the unreacted gas discharged at a high temperature from the fuel cell can advantageously be collected and reused through the humidifier.
Typically, a humidifier for fuel cell comprises a membrane housing in which a bundle of hollow fiber membranes are integrated to supply the moisture to the reaction gas flowing through a hollow; an inlet for introducing unreacted gas of high humidity; and an outlet for discharging the gas.
However, the hollow fiber membrane used for the humidifier according to the related art can not contain the sufficient moisture due to the structural reason. Thus, if a gas inflow speed is raised in a moment due to a sudden acceleration while an automobile is driven, it is impossible to promptly humidifying the reaction gas. As a result, insufficiently-humidified gas is supplied to the fuel cell from the humidifier causing the instantaneous decrease in output of a fuel cell system.
In order to overcome this kind of problem, there has been proposed a method of increasing a thickness of the hollow fiber membrane so as to increase the humidity content in the hollow fiber membrane. However, the increased thickness of the hollow fiber membrane causes difficulties in moisture transfer so that it might cause adverse effect such as deterioration of humidifying performance.